1. Field of the Invention
The present invention relates to a method of making shell moulds for castings, and is particularly concerned with the manufacture of such moulds for casting processes of the "lost wax" type.
2. Summary of the Prior Art
As is well known, lost wax casting processes using ceramic shell moulds generally comprise the following stages:
making a pattern, usually from wax, of the article which is to be cast;
deposition of a ceramic suspension on the pattern from a thick slip;
deposition of ceramic particles in stucco form;
drying, for example in an oven, to achieve removal of solvent from the suspension, and consolidation of the ceramic layer;
deposition of a further ceramic suspension layer from a fluid slip;
further deposition of ceramic particles in stucco form;
drying as before;
addition of further ceramic layers as necessary to introduce new elements and to obtain the desired mould characteristics for the particular application, be they of a physical, chemical or mechanical nature;
heat treatment of the resulting assembly under specific conditions, adapted to the compositions and products used, leading to the removal of the organic pattern and to a "firing" and consolidation of the mould, in particular by sintering, which may be reactive; and
casting the article in the mould so obtained.
Examples of slip composition are given in particular in French Patent 2 599 649.
In the course of using processes such as are generally described above various difficulties have arisen, necessitating special precautions during use. The deposition of the first layer on the wax pattern is found to be a particularly delicate operation. Indeed, it is necessary to use a thick slip to obtain this first layer, to coat it with a deposit of particles and to watch closely the drying conditions to prevent overdrying. All these restricting conditions are necessary to limit the decoherence of the said first layer and to prevent flaking. These difficulties are aggravated particularly when the pattern has areas of limited accessibility, termed "enclosed", and problems of adherence of the ceramic deposit on the wax pattern leading to detachment of the layer appear also at sharp angles or fine sections. These configurations are encountered especially in applications particularly envisaged by the invention, i.e. aircraft components, especially for aeroengines, which have these features, such as, for example, cooled turbine blades.
Attempts at a solution to the problem have generally involved adjusting the compositions of the slips. For example, in the case of known compositions comprising a colloidal binder, surface-active agents, such as nonylphenol based substances, have been added. However, these additions involve a risk of destabilizing the compositions and usually produce a foaming effect, which requires a further addition of an anti-foaming agent.